Referring to FIG. 1a, in an optical communications system, a transmitter 2 typically comprises a signal generator 4 for converting a digital signal X(n) to be transmitted into a drive signal S(t) which drives a modulator 6 (such as, for example, an Mach-Zehnder modulator (MZM) so as to modulate a narrow-band optical carrier generated by a laser 8 to generate a corresponding optical channel signal, which may then multiplexed into an optical fiber link for transmission through the optical communications system to a receiver. Typically, the drive signal S(t) is a radio frequency (RF) analog electrical signal. In such cases, the signal generator 4 typically includes a digital signal processor (DSP) 10 cascaded with a digital-to-analog converter (DAC) 12. The DSP 10 operates to process the digital signal X(n) to generate a corresponding digital drive signal X′(m) which is designed in accordance with the performance and operating requirements of the DAC 12. The DAC 12 operates in a conventional manner to convert the digital drive signal X′(m) into the required analog RF drive signal S(t) for modulation onto the optical carrier.
In simple cases, the processing implemented by the DSP 10 will typically include de-serializing a serial digital signal X(n) to generate the digital drive signal X′(m) as a parallel digital sample stream; adjusting a number of bits resolution of the digital drive signal X′(m) to match that of the DAC input; and adjusting a timing of the digital drive signal X′(m) to match the sample rate of the DAC. For example, consider an arrangement in which the signal generator 4 receives a serial digital signal X(n) comprising QPSK encoded data. In this case, the DSP 10 may deserialize the received serial bit stream to generate into a two-bit parallel digital drive signal X′(m) so that the DAC 12 will output a four-level analog drive signal S(t). In additional, the DSP 10 may adjust signal timing so that bits of the serial digital signal X(n) can be latched into the DSP 10 at a timing of the input digital signal X(n), and words of the two-bit parallel digital drive signal X′(m) are latched into DAC 12 at the sample rate Fs of the DAC 12.
In more complex arrangements, the DSP 10 may also apply a desired encoding scheme to the digital signal X(n). Various encoding schemes may be used as desired, including but not limited to On-Off Keying (OOK), Phase Shift Keying (PSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM) and Forward Error Correction (FEC). The DSP 10 may also apply a compensation function (i.e. pre-distortion) to the digital drive signal X′(m) to compensate impairments of the optical communications link, such as, for example, dispersion.
As channel bit rates rise to 100 Giga-bits per second (Gb/s) and beyond, high speed signal generators are required to generate the necessary electrical drive signals for modulation. High speed signal generators may also be useful in other fields, such has high speed wireless communications.
A limitation of known techniques is that, for a DAC having a sample rate of Fs, the frequency spectrum of the output signal S(t) consists of a baseband symmetrically distributed about DC (0 Hz) and extending to ±Fs/2, and corresponding harmonic bands symmetrically distributed about ±nFs (where n is an integer). These harmonic bands are duplicates of the baseband, and so do not contribute to the bit-rate of the optical channel. For example, consider a scenario in which the DAC is supplied with a digital drive signal X′(m) defining a pure sinusoid having a frequency of fo. The frequency spectrum of the analog output of the DAC will contain baseband frequency components at ±fo, and first harmonic frequency components at −Fs±fo and at Fs±fo, as may be seen in FIG. 1b. In order to avoid aliasing, a low-pass filter characteristic 14 having a cut-off frequency of Fs/2 is typically used to attenuate the harmonic bands, leaving only the baseband components in the drive signal S(t) output from the signal generator.
As is well known in the art, the analog drive signal S(t) output from the signal generator is inherently limited to frequency components between DC (0 Hz) and 1/2  of the sample rate Fs of the DAC. For example, a DAC having a sample rate Fs=10 Giga-Samples per second (GS/s) can generate an analog output signal S(t) with frequency components between DC and Fs/2=5 GHz. In a system that utilizes Nyquist sampling (2 samples per symbol), such a DAC would be suitable for use in a signal generator that operates at a maximum symbol rate of 5 Giga-baud (Gbaud). In the case of QPSK encoding (2 bits per symbol), this translates into a maximum channel bit rate of 10 Gb/s. In high speed optical communications systems, the sample rate of the DAC used to generate the analog drive signal may be the limiting factor in determining the maximum bit-rate of an optical channel.
The highest speed commercially available DAC currently known to the inventors has a maximum sample rate of Fs=20 GS/s (available from Tektronix). This allows for the generation of an analog output drive signal with spectral content from DC to 10 GHz, and supports symbol rates of up to 10 Gbaud with Nyquist sampling. A high speed signal source is needed that can generate signals with spectral content from DC to at least 20 GHz, and further increases in spectral width are expected to be needed in the future.
Techniques and high speed signal sources for generating high bandwidth signals remain highly desirable.